About Grsecurity

The Grsecurity Project

The grsecurity project, hosted on http://www.grsecurity.org, provides various patches to the Linux kernel which enhance your system's overall security. The various features brought by grsecurity are discussed in the next chapter; a comprehensive list is maintained on the grsecurity features page itself.

As grsecurity's features are mostly kernel-based, the majority of this document explains the various kernel features and their respective sysctl operands (if applicable).

Kernel Configuration

Throughout this document we will talk about kernel configuration using the kernel variables like CONFIG_GRKERNSEC_PAX_NO_ACL_FLAGS. These are the variables that the kernel build process uses to determine if a certain feature needs to be compiled.

When you configure your kernel through make menuconfig or similar, you receive a user interface through which you can select the various kernel options. If you select the Help button at a certain kernel feature you will see at the top that it lists such a kernel variable.

You can therefore still configure your kernel as you like - with a bit of thinking. And if you can't find a certain option, there's always the possibility to edit /usr/src/linux/.config by hand :)

PaX

Fighting the Exploitation of Software Bugs

PaX introduces a couple of security mechanisms that make it harder for attackers to exploit software bugs that involve memory corruption (so don't treat PaX as if it protects against all possible software bugs). The PaX introduction document talks about three possible exploit techniques:

introduce/execute arbitrary code

execute existing code out of original program order

execute existing code in original program order with arbitrary data

One prevention method disallows executable code to be stored in writable memory. When we look at a process, it requires five memory regions:

a data section which contains the statically allocated and global data

a BSS region (Block Started by Symbol) which contains information about the zero-initialized data of the process

a code region, also called the text segment, which contains the executable instructions

a heap which contains the dynamically allocated memory

a stack which contains the local variables

The first PaX prevention method, called NOEXEC, is meant to give control over the runtime code generation. It marks memory pages that do not contain executable code as non-executable. This means that the heap and the stack, which only contain variable data and shouldn't contain executable code, are marked as non-executable. Exploits that place code in these areas with the intention of running it will fail.

If you are running a non-x86 system you will observe that there is no CONFIG_GRKERNSEC_PAX_NOEXEC. You should select CONFIG_GRKERNSEC_PAX_PAGEEXEC instead as it is the only non-exec implementation around.

Controlling PaX

Not all Linux applications are happy with the PaX security restrictions. These tools include xorg-x11, java, mplayer, xmms and others. If you plan on using them you can elevate the protections for these applications using chpax and paxctl. You can find they on AUR.

You can also use pax-utils, which is a small toolbox which contains useful applications to administrate a PaX aware server. Find it on AUR.

Interesting tools include scanelf and pspax:

With scanelf you can scan over library and binary directories and list the various permissions and ELF types that pertain to running an ideal pax/grsec setup

With pspax you can display PaX flags/capabilities/xattr from the kernel's perspective

Verifying the PaX Settings

Peter Busser has written a regression test suite called paxtest. This tool will check various cases of possible attack vectors and inform you of the result. When you run it, it will leave a logfile called paxtest.log in the current working directory.

strcpy and memcpy are listed as Vulnerable. This is expected and normal - it is simply showing the need for a technology such as ProPolice/SSP

there is no randomization for PAGEEXEC. This is normal since our recommended x86 kernel configuration didn't activate the PAGEEXEC setting. However, on arches that support a true NX (non-executable) bit (most of them do, including x86_64), PAGEEXEC is the only method available for NOEXEC and has no performance hit.

RBAC

Role Based Access Control

There are two basic types of access control mechanisms used to prevent unauthorized access to files (or information in general): DAC (Discretionary Access Control) and MAC (Mandatory Access Control). By default, Linux uses a DAC mechanism: the creator of the file can define who has access to the file. A MAC system however forces everyone to follow rules set by the administrator.

The MAC implementation grsecurity supports is called Role Based Access Control. RBAC associates roles with each user. Each role defines what operations can be performed on certain objects. Given a well-written collection of roles and operations your users will be restricted to perform only those tasks that you tell them they can do. The default "deny-all" ensures you that a user cannot perform an action you haven't thought of.

Configuring the Kernel

Working with gradm

gradm is a tool which allows you to administer and maintain a policy for your system. With it, you can enable or disable the RBAC system, reload the RBAC roles, change your role, set a password for admin mode, etc.

When you install gradm a default policy will be installed in /etc/grsec/policy. Please see in AUR for gradm package.

By default, the RBAC policies are not activated. It is the sysadmin's job to determine when the system should have an RBAC policy enforced. Before activating the RBAC system you should set an admin password.

Generating a Policy

The RBAC system comes with a great feature called "learning mode". The learning mode can generate an anticipatory least privilege policy for your system. This allows for time and money savings by being able to rapidly deploy multiple secure servers.

To use the learning mode, activate it using gradm:

# gradm -F -L /etc/grsec/learning.log

Now use your system, do the things you would normally do. Try to avoid rsyncing, running locate of any other heavy file i/o operation as this can really slow down the processing time.

When you believe you have used your system sufficiently to obtain a good policy, let gradm process them and propose roles under /etc/grsec/learning.roles:

# gradm -F -L /etc/grsec/learning.log -O /etc/grsec/learning.roles

Audit the /etc/grsec/learning.roles and save it as /etc/grsec/policy (mode 0600) when you are finished.

This is the result of the | operator which takes both sets and returns the files that exist in either set. If a file exists in both sets, it is returned as well and the mode contains the flags that exist in either set.

subject /somebinary o
$objset1 - $somename2

This example would expand to:

subject /somebinary o
/root/blah rw
/root/blah2 h
/root/blah3 x

This is the result of the - operator which takes both sets and returns the files that exist in the set on the left but not in the match of the file in set on the right. If a file exists on the left and a match is found on the right (either the filenames are the same, or a parent directory exists in the right set), the file is returned and the mode of the second set is removed from the first set, and that file is returned.

Process Restrictions

Executable Protection

With grsecurity you can restrict executables. Since most exploits work through one or more running processes this protection can save your system's health.

Network Protection

Linux' TCP/IP stack is vulnerable to prediction-based attacks. grsecurity includes randomization patches to counter these attacks. Apart from these you can also enable socket restrictions, disallowing certain groups network access alltogether.

Kernel Settings

The following kernel settings enable various executable and network protections: